1. Field of the Invention
The present invention relates to a control apparatus that performs vector control an inverter for driving an induction motor and, in particular, to improvements in the precision and response of the speed control thereof.
2. Description of the Related Art
FIG. 5 is a block diagram illustrating a conventional control apparatus for an inverter which drives an induction motor by converting a DC power to an AC power of an arbitrary frequency and voltage. In FIG. 5, numeral 1 denotes an inverter, 2 denotes a motor driven by the inverter 1, 3 denotes a speed detector for detecting the rotational speed of the motor 2, 4 denotes a current detector for detecting two phases of the output current of the inverter 1, e.g., a U-phase current i.sub.U and a V-phase current i.sub.V.
Numeral 5 denotes a current coordinate converter which converts currents i.sub.U and i.sub.V detected by the current detector 4 to a torque current i.sub..gamma. and an exciting current i.sub..delta. in current coordinates. Numeral 6 denotes a computing unit which computes the output frequency .omega..sub.1 of the inverter 1 on the basis of a speed set value .omega..sub.m * of the motor 2, a torque current i.sub..gamma. and an exciting current i.sub..delta. sent from the current coordinate converter 5, and a speed detection value .omega..sub.m detected by the speed detector 3, and computes and outputs a torque voltage v.sub..gamma. *, an exciting voltage v.sub..delta. *, and a phase .theta..sub.1 of the output voltage.
Numeral 7 denotes a voltage coordinate converter which determines three-phase output voltages v.sub.u, v.sub.v, and v.sub.w on the basis of the torque voltage v.sub..gamma. *, exciting voltage v.sub..delta. * and phase .theta..sub.1 sent from the computing unit 6. Numeral 8 denotes a PWM controller which produces switching signals for turning the switching elements of the inverter 1 on or off by multiplying a DC bus line voltage Vd by the output voltages v.sub.u, v.sub.v, and v.sub.w sent from the voltage coordinate converter 7.
The basic rules of vector control in the computing unit 6 of the control apparatus configured as described above conform to the following four equations: EQU v.sub..gamma. *=K(i.sub..gamma. *-i.sub..gamma.)+R.sub.1 i.sub..gamma. +L.sub.1 .omega..sub.1 i.sub..gamma. ( 1) EQU V.sub..delta. *=K(i.sub..delta. *-i.sub..delta.)+R.sub.1 i.sub..delta. -.sigma.L.sub.1 .omega..sub.1 i.sub..gamma. ( 2) EQU i.sub..gamma. *=K.sub.m (.omega..sub.m *-.omega..sub.m) (3) EQU .omega..sub.1 =n.omega..sub.m +R.sub.2 i.sub..gamma. /L.sub.2 i.sub..delta. * (4)
where K is an amplification gain of the difference between the torque current set value i.sub..gamma. * and the torque current i.sub..gamma. and of the difference between the exciting current set value i.sub..delta. * and the exciting current i.sub..delta., K.sub.m is an amplification gain of the difference between the speed set value .omega..sub.m * and the speed detection value .omega..sub.m, R.sub.1 is a primary resistance value of the motor 2 which is a load, R.sub.2 is a secondary resistance value of the motor 2, L.sub.1 is a primary reactance of the motor 2, .sigma. is a current leakage coefficient (.sigma.=1-M.sup.2 /L.sub.1 L.sub.2), and n is the number of pole pairs of the motor 2.
FIG. 6 is a block diagram illustrating the computing unit 6 configured according to the above-mentioned basic rules. In FIG. 6, numerals 31 to 34 denote subtracters, numerals 41 to 44 denote adders, 51 to 59 denote amplifiers, 61 and 62 denote multipliers, 71 denotes a divider, 81 denotes an integrator, and 91 denotes an exciting current setter.
The operation of the inverter control apparatus configured as described above will be explained below.
When currents i.sub.u, i.sub.v detected by the current detector 4 are sent to the current coordinate converter 5, the current coordinate converter 5 performs a calculation shown below to compute the torque current i.sub..gamma. and exciting current i.sub..delta. and sends these to the computing unit 6. ##EQU1##
where .theta..sub.1 =.omega..sub.1 t.
In FIG. 6, the computing unit 6 computes, using the subtracter 31, the difference between the speed set value .omega..sub.m * and the speed detection value .omega..sub.m of the motor 2 which is detected by the speed detector 3, and then computes the torque current set value i.sub..gamma. * shown in equation (3) by multiplying the computed difference (.omega..sub.m *-.omega..sub.m) by gain K.sub.m.
The exciting current computed value i.sub..delta. * is set in correspondence to the speed of the motor 2. That is, the speed detection value .omega..sub.m is sent to the exciting current setter 91 by means of which the exciting current set value i.sub..delta. * is calculated.
The output frequency .omega..sub.1 of the inverter 1 shown in equation (4) is computed as follows: First, the speed detection value .omega..sub.m is multiplied by n by means of the amplifier 53 to compute n.omega..sub.m shown in term 1 of equation (4). Next, the torque current i.sub..gamma. sent from the current coordinate converter 5 is multiplied by gain R.sub.2 /L.sub.2 by means of the amplifier 55 and is sent to the divider 71. The divider 71 computes the secondary frequency R.sub.2 i.sub..gamma. /L.sub.2 i.sub..delta. * shown in term 2 of equation (4) on the basis of the R.sub.2 i.sub..gamma. /L.sub.2 sent and the exciting current set value i.delta.* from circuit 92. This secondary frequency R.sub.2 i.sub..gamma. /L.sub.2 i.sub..delta. * is added to n.omega..sub.m by the adder 43 to compute the output frequency .omega..sub.1 of the inverter 1.
The difference (i.sub..gamma. *-i.sub..gamma.) between the torque current set value i.sub..gamma. * and the torque current i.sub..gamma. is computed by means of the subtracter 32. This difference (i.sub..gamma. * -i.sub..gamma.) is multiplied by gain K by means of the amplifier 52 to compute the voltage component K (i.sub..gamma. * i.sub..gamma.) shown in term 1 of equation (1). At the same time, the torque current i.sub..gamma. is multiplied by gain R.sub.1 by means of the amplifier 54 to compute the voltage component R.sub.1 i.sub..gamma. shown in term 2 of equation (1). Meanwhile, the exciting current i.sub..delta. sent from the current coordinate converter 5 is multiplied by gain L.sub.1 by means of the amplifier 57, and this L.sub.1 i.sub..delta. is multiplied by the output frequency .omega..sub.1 by means of the multiplier 61, with the result that L.sub.1 .omega..sub.1 i.sub..delta. shown in term 3 of equation (1) is computed. The torque voltage v.sub..gamma. * shown in equation (1) is determined by adding these voltage components by means of adders 41 and 42.
Meanwhile, in the same manner as for the torque voltage v.sub..gamma. *, the voltage component of each term of equation (2) is computed using the exciting current set value i.sub..delta. *, the exciting current i.sub..delta., the torque current i.sub..gamma., and the output frequency .omega..sub.1 by means of the subtracter 33, the amplifiers 59, 58 and 56, and the multiplier 62, and is added or subtracted by means of the adder 44 or the subtracter 34 to determine the exciting voltage v.sub..delta. * shown in equation (2).
The output frequency .omega..sub.1 is sent to the integrator 81 where it is integrated with respect to time, and the voltage phase .theta..sub.1 represented by .theta..sub.1 =.intg..omega..sub.1 dt is determined.
The torque voltage v.sub..gamma. *, the exciting voltage v.sub..delta. * and the voltage phase .theta..sub.1 determined by means of the computing unit 6 are sent to the voltage coordinate converter 7 where the following equation is performed to compute three-phase output voltages v.sub.u, v.sub.v, and v.sub.w. ##EQU2##
These output voltages v.sub.u, v.sub.v, and v.sub.w are sent to the PWM controller 8 shown in FIG. 3, in which an on/off signal for each switching element of the inverter 1 is produced.
By performing vector control for the inverter 1 as described above, the .gamma.-axis (torque axis) component .lambda..sub..gamma. of the secondary interlinkage magnetic flux of the motor 2 becomes zero, and the .delta.-axis (exciting axis) component .lambda..sub..delta. becomes constant. As a result, the output torque .tau. of the motor 2 can be rendered proportional to the product of the exciting current set value i.sub..delta. * and the torque current i.sub..gamma. and thus excellent control performance can be obtained.
FIG. 7 is a block diagram illustrating a conventional inverter control apparatus of a type in which no speed detector is used. FIG. 8 is a block diagram illustrating a computing unit used in the control apparatus in FIG. 7. Elements in FIGS. 7 and 8 which correspond to essentially the same elements in FIGS. 5 and 6 are identified by the same reference designations, and an explanation of the construction thereof is omitted to avoid duplication of the same. A point of difference between FIG. 7 and the example of the prior art shown in FIG. 5 is that the speed detector 3 for detecting the rotational speed of the motor 2 is not disposed in the motor 2. A point of difference between the computing unit 6A and the computing unit 6 shown in FIG. 6 is that an induction motor simulator 10x is disposed. This induction motor simulator 10x estimates the speed .omega..sub.m of the motor 2 on the basis of the torque current i.sub..gamma. and the exciting current i.sub..delta., on which coordinate conversion has been performed by the current coordinate converter 5, and the DC bus line voltage Vd. An example of the estimating method is presented in "Speed sensorless vector control of induction motor in which model range adaptation system is applied" presented in Part D, Electrical Society thesis (issued Mar. 1988).
In this example of the prior art, regarding the torque current set value i.sub..gamma. *, the difference between the speed set value .omega..sub.m * and the speed estimation value .omega..sub.m is computed by means of the subtracter 31. The computed difference (.omega..sub.m *-.omega..sub.m) is multiplied by gain K.sub.m by means of the multiplier 51 to compute the torque current set value i.sub..gamma. *. Of the above-mentioned four equations showing the basic rules of vector control, equations (3) and (4) are expressed in the following manner: EQU i.sub..gamma. *=K.sub.m (.omega..sub.m *-.omega..sub.m) (7) EQU .omega..sub.1 =n.omega..sub.m +R.sub.2 i.sub..gamma. /L.sub.2 i.sub..gamma. I (8)
The exciting current set value i.sub..delta. * is set by means of the exciting current setter 91 on the basis of the speed estimation value .omega..sub.m of the motor 2.
In addition, regarding the output frequency .omega..sub.1 of the inverter 1, the speed estimation value .omega..sub.m of the motor 2 is multiplied by n by means of the amplifier 53 to compute the n.omega..sub.m shown in term 1 of equation (8).
As described above, in this example of the prior art, the speed estimation value .omega..sub.m of the induction motor simulator 10x is used in place of the speed detection value .omega..sub.m by means of the speed detector 3 of the prior art shown in FIGS. 5 and 6. The subsequent operations are essentially the same as the operations of the example of the prior art shown in FIGS. 5 and 6.
However, when the motor 2 driven by an inverter is controlled by vector control with high precision as mentioned above, the constants of the motor 2, for example, a primary resistance R.sub.1 and a secondary resistance R.sub.2 used in the above calculations, must be in agreement with the actual resistance values of the motor 2. In particular, when the secondary resistance R.sub.2 varies due to a rise in temperature or the like of the motor 2, the output frequency .omega..sub.1, the torque voltage v.sub..gamma. *, and the exciting voltage v.sub..delta. * cannot follow the variation, thus causing the performance of accurate vector control to be impossible. For this reason, in spite of the fact that the exciting current set value i.sub..delta. * is constant, the .delta.-axis component .lambda..sub..delta. of the secondary interlinkage magnetic flux will not be constant, and the .gamma.-axis component .lambda..sub..gamma. will thus not be zero. Therefore, the output torque .tau. of the motor, which is one target of the vector control, can not be rendered proportional to the product of the exciting current set value i.sub..delta. * and the torque current i.sub..gamma.. This causes a drawback in that control performance is degraded.
In the case of vector control under which no speed detector is used, since the action of the motor 2 is simulated on the basis of the constant itself of the motor 2 by using the induction motor simulator 10x, the speed estimation on value .omega..sub.m deviates in the same manner as above in the case where the actual constant value of the motor 2 is not in agreement with that of the simulator. Thus, a disadvantage exists in that the entire system will not be stable and the system will be operated at a greatly deviated speed.